Leadless chip carriers are packages for mounting integrated circuit chips with respect to printed wiring boards. Developed to improve upon the DIP (dual in-line package), the leadless chip carrier replaces the outwardly protruding leads of the DIP with electrically conductive pads distributed about the periphery of the chip carrier. In this manner the leadless chip carrier provides an equivalent number of electrical contacts using significantly less substrate area. The chip carrier pads further form shorter and more reliable conductive paths with significantly reduced inductance and compacitance, allowing the chip carrier to handle higher frequency currents. The shorter paths also reduce signal transmission time.
A difficulty encountered, particularly when using ceramic leadless chip carriers, is in mounting the chip carrier to a conventional printed wiring board, especially in an environment of temperature extremes. This is due to the difference in thermal expansion coefficient between the ceramic chip carrier and the printed wiring board, commonly known as thermal mismatch. When the printed wiring board and chip carriers mounted thereto experience temperature extremes, the different rates at which they expand or contract causes stress concentrations at the interfacing solder joints. Repeated expansions and contractions (cycles) can cause cracking at these joints and increases electrical resistance to unacceptable levels. Thermal mismatch can hinder direct chip carrier/PWB attachment due to the difference between temperatures required for vapor phase sodering (e.g., 450.degree. F.) and normal operating temperatures.
Attempts to solve the thermal mismatch problem include special sockets for containing the chip carrier, the sockets in turn being mounted to the printed wiring board. Aside from the added expense, these sockets take up added space on the printed wiring board and increase conductive path lengths. Another approach is to select materials more thermally compatible with the ceramic chip carrier. However, the expense has been prohibitive.
A further problem with many of the special sockets is that they require space between the leadless chip carrier and the PWB. Consequently, chip carriers must be cooled by convection, a method which often requires blowers or other air moving equipment, and is unsatisfactory in many uses, particularly in certain military applications.
It is therefore an object of this invention to provide apparatus for mounting a chip carrier directly to a printed wiring board to withstand repeated thermal cycles over a wide temperature range, with no degradation or damage to the mounting apparatus, PWB or chip carrier. It is a further object of this invention to provide a simple and convenient method for mounting leadless chip carriers directly to a PWB to enable conduction cooling. Yet another object of the invention is to provide a universal connector assembly adaptable to multiple sizes of chip carriers.